Vacuum toilets having rinse fluid systems are generally known in the art. Such vacuum toilets generally include a bowl having an outlet connected by a discharge pipe to a vacuum source, which generates a vacuum level in the discharge pipe. A discharge valve disposed between the toilet outlet and the discharge pipe controls when vacuum is present in the toilet bowl. When the valve is open, a pressure differential created by the vacuum at the bowl outlet and atmospheric pressure inside the toilet bowl pushes material present in the bowl into the discharge pipe.
It is common for such vacuum toilets to provide a rinse fluid system which rinses the surface of the bowl while the discharge valve is open. The rinse fluid systems typically include a spray ring which extends around the rim of the toilet bowl and is formed with nozzles. The nozzles are positioned so that the entire inside surface of the bowl is rinsed when the spray ring is connected to a source of rinse fluid. A problem associated with such rinse fluid systems is the build-up of deposits in the nozzles. Certain materials, such as calcium carbonate, are dissolved in the rinse water. When rinse water remains in the nozzle, it eventually evaporates, leaving a calcium carbonate residue in the toilet bowl and the nozzles. Consequently, the surface of the toilet bowl may become roughened thereby reducing the efficiency with which waste is removed during the flushing operation. In addition, the residue may clog the nozzles, thereby causing incomplete rinsing of the toilet bowl.
In many vacuum toilet system applications, very little rinse fluid is used with each flush, and a very large volume of air passes through the system during each flush. As a result, a hard plaque may build up on the internal surfaces of the discharge pipe and holding tank. While the plaque may be removed using acid, it may not be safe to do so in certain applications, such as vacuum toilets used on aircraft.
It is further known to introduce a chemical reagent into the rinse fluid of a conventional flush toilet for sanitizing the toilet bowl at each flush. This may be done by hanging a cake of water-soluble material in the toilet tank. However, to the best of applicants' knowledge, a similar technique has never been applied to vacuum toilet systems, due to the need for an electric pump to introduce the chemical reagent into the rinse fluid. The need for an additional electrical outlet is a particular disadvantage with respect to retrofit applications such as on aircraft. Further, the electric pump would require controls, which would also require modification of the flush control unit of the vacuum toilet.
Commonly owned U.S. Pat. No. 5,692,250 to Oldfelt et al. discloses a vacuum toilet system in which a chemical reagent is introduced into a rinse water supply during each flush. The system uses a pressure-actuated pump which operates in response to the presence of pressurized rinse fluid, thereby to inject chemical reagent into the stream of rinse of fluid. As a result, the puLnp does not require additional electrical outlets or control lines.
While this system generally addresses many of the problems outlined above, applicants have found this system difficult to implement. More specifically, applicants have found that the check valves used to introduce the chemical reagent into the rinse fluid stream are susceptible to collapse under the pressure of the rinse fluid, thereby rendering reagent injection difficult. In addition, the chemical injection pump is susceptible to losing its prime due to the collection to air bubbles both in a pump chamber and upstream of the check valves. As described in the '250 patent, the pump includes a dual-headed piston disposed inside a housing. When the piston moves in a first direction, it draws reagent into a small diameter portion of the housing. When the piston moves in a second direction, the reagent in the small diameter portion is ejected into the rinse fluid stream. Air bubbles entrapped in the reagent may be drawn toward the chemical pump during operation. The air bubbles may aggregate at irregular surfaces in the reagent supply pipe, such as at shoulders, ledges, and corners, to form an air pocket which causes hydraulic lock of the reagent. In addition, air bubbles passing through the first check valve may collect in the pump chamber to form another air pocket. The air pocket inside the pump chamber may be so large that it is not purged through the second check valve with a single stroke of the pump, thereby causing the pump to lose its prime.